Heterojunction bipolar transistor and method for fabricating the same

ABSTRACT

A heterojunction bipolar transistor includes an emitter layer, a base layer and a collector layer laminated on a top surface of a semiconductor substrate, and a heat sink layer made of a metal and provided on a rear surface of the substrate. A via hole is cut through the emitter layer, the base layer, the collector layer and the substrate. A surface electrode of the emitter layer and the heat sink layer are connected to each other by a metal wiring line running through within the via hole, which is capable of improving the heat radiation and reducing the emitter inductance.

BACKGROUND OF THE INVENTION

The present invention relates to heterojunction bipolar transistorsincluding those connected in parallel and methods for fabricating thesame. The invention also relates to a high-frequency transmitter orreceiver having a heterojunction bipolar transistor as an amplifier.

As a high-power device for microwave band, there has been developedGaAs-based heterojunction bipolar transistors (hereinafter, referred toas HBTs). Generally, HBTs, which are high in thermal resistance, have aproblem that when used as a high-power device, HBTs would involve highjunction temperature. On this account, as shown in FIG. 22, a structurefor improving heat radiation property has been proposed in JapanesePatent Laid-Open Publication HEI 8-279562. FIG. 22A shows a planarpattern of HBTs connected in parallel for high-power operation, and FIG.22B shows a cross section taken along a line B—B of FIG. 22A. In thisstructure, a plurality of HBTs 90 each having a collector electrode 106,a base electrode 105 and an emitter electrode 104 are included on thesurface side of a GaAs substrate 113, where via holes 110 are providedbetween adjacent HBTs 90 so as to be cut through the substrate from itstop to rear surface side. Heat generated at a junction 127 on the topsurface side of each HBT 90 is conducted from the emitter electrode 104of the transistor to a metal body 99 within its adjacent via holes 110via an air bridge 111, and further conducted from the metal body 99 to aplated heat sink (hereinafter, referred to as PHS) layer 112 provided onthe substrate rear surface, thus being radiated.

However, this conventional structure has a first drawback that forimplementation of even higher power output, electric resistance of theair bridge 111 is not negligible, with heat radiation effectinsufficient, so that the junction temperature inside the transistorcannot be reduced sufficiently. In this conventional structure, there isa second drawback that because of limitations in reducing the emitterinductance, there may arise variations in high frequency characteristicsor the gain in high frequency operation may decline.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide aheterojunction bipolar transistor (including those connected inparallel) which is capable of improving the heat radiation and reducingthe emitter inductance.

Another object of the invention is to provide a fabricating methodcapable of fabricating such a heterojunction bipolar transistor.

A further object of the invention is to provide a high-frequencytransmitter or receiver having such a heterojunction bipolar transistoras an amplifier.

In order to achieve the above-mentioned object, the present inventionprovides a heterojunction bipolar transistor comprising: an emitterlayer, a base layer and a collector layer laminated on a top surface ofa semiconductor substrate; and a heat sink layer made of a metal andprovided on a rear surface of the substrate, wherein a via hole is cutthrough the emitter layer, the base layer, the collector layer and thesubstrate, and a surface electrode of the emitter layer and the heatsink layer are connected to each other by a metal wiring line runningthrough within the via hole.

In the heterojunction bipolar transistor of this invention, heatgenerated during operation at junctions (mainly, an interface betweenthe base layer and the collector layer) on the top surface side of thesemiconductor substrate is dissipated through two paths. One of thepaths is a path along which the heat conducts from the junction via thesurface electrode of the emitter layer to the metal wiring line on thesubstrate top surface side, and further conducts from the metal wiringline within the via hole to the heat sink layer on the substrate rearsurface side. The other path is a path along which heat conducts fromthe junction via an interior of the substrate to the metal wiring linewithin the via hole, and further conducts from there to the heat sinklayer on the substrate rear surface side. Since the heat generated atthe junction is dissipated through two paths as described above, heatradiation property of the heterojunction bipolar transistor is improved.Also, since the via hole extends through the emitter layer, the baselayer, the collector layer and the substrate, the surface electrode ofthe emitter and the top surface of the via hole are very close to eachother. Therefore, the metal wiring line is led from the surfaceelectrode of the emitter layer into the via hole at a very shortdistance. As a result, emitter inductance is reduced and high-frequencycharacteristics are improved, as compared with the case where an airbridge is used.

In an embodiment of the invention, the via hole has a cross sectionformed into a polygonal shape in which apex angles are obtuse angles, ora circular shape.

When the cross-sectional shape of the via hole has acute angles, thereis a possibility that electric field concentration may occur at theacute-angle portions during operation, causing the device reliability tolower. Thus, in the heterojunction bipolar transistor of thisembodiment, the via hole has a cross section formed into a polygonalshape in which apex angles are obtuse angles, or a circular shape. As aresult, electric field concentration around the via hole is suppressed.Therefore, the device reliability is improved.

In an embodiment of the invention, an interior of the via hole is buriedwith a same material as that of the metal wiring line.

In the heterojunction bipolar transistor of this embodiment, since theinterior of the via hole is buried with the same material as that of themetal wiring line, the heat radiation effect through the via hole isenhanced, so that the heat radiation property is further improved. As aresult, stabler device characteristics as well as higher devicereliability can be obtained.

In an embodiment of the invention, a peripheral edge portion of theemitter layer is formed so as to be thinner in thickness than residualportion of the emitter layer.

In the heterojunction bipolar transistor of this embodiment, thethickness of the peripheral edge portion of the emitter layer is thinnerthan the thickness of the residual portion of the emitter layer, thatis, what we called an edge-thinning structure is formed. Therefore,re-combination of holes and electrons generated between the peripheraledge portions of the emitter layer and the base layer during operationis prevented. As a result, the device reliability can be enhanced.

In an embodiment of the invention, a plurality of heterojunction bipolartransistors are arrayed on a common semiconductor substrate andelectrically connected to one another so as to be enabled to operate inparallel.

In this parallel-connected heterojunction bipolar transistors, since anyof the heterojunction bipolar transistors are electrically connected toone another so as to be enabled to operate in parallel, a high-poweroutput operation is enabled. Also, heat generated at the junction ofeach transistor is dissipated to the heat sink layer on the substraterear surface. Therefore, heat concentration due to performancevariations among the transistors is suppressed so that the reliabilityis improved.

In an embodiment of the invention, a groove extending from the topsurface of the substrate to the rear surface of the substrate isprovided in the common semiconductor substrate so as to partitionadjacent heterojunction bipolar transistors from one another.

Generally, in parallel-connected heterojunction bipolar transistors,adjacent transistors would thermally affect one another duringoperation. When one transistor is unequally heated with the result ofnonuniform heat generation, a transistor adjacent to the transistor isaffected with the result of heat generation, which in some extreme casesleads to breakage. Also, when no transistor is present in adjacency toone transistor, there is a possibility that the thermal balancecollapses, leading to a similar result. Therefore, in theparallel-connected heterojunction bipolar transistors of thisembodiment, a groove which extends from top surface to rear surface ofthe substrate is provided in the common semiconductor substrate so as topartition adjacent heterojunction bipolar transistors from each other.As a result, adjacent heterojunction bipolar transistors are thermallyshielded from each other during operation, thus never affecting eachother thermally. Moreover, the transistors are uniformized in heatcapacity, thus operating uniformly. Therefore, the device reliabilitycan be improved.

The present invention also provides a method for fabricating aheterojunction bipolar transistor, comprising the steps of: laminating acollector layer, a base layer and an emitter layer in this order on atop surface side of a semiconductor substrate; patterning the collectorlayer, the base layer and the emitter layer so that an area of an upperlayer among the collector layer, the base layer and the emitter layermay become smaller; forming a surface electrode for ohmic contact oneach surface portion of the collector layer, the base layer and theemitter layer; forming a first via hole which extends through theemitter layer, the base layer and the collector layer and ends at aspecified depth within the substrate; forming a metal wiring line whichextends from the surface electrode of the emitter layer to within thefirst via hole so as to reach a bottom portion of the first via hole;polishing a rear surface side of the substrate up to the bottom portionof the first via hole; and providing a heat sink layer made of a metalon the polished rear surface of the substrate so that the heat sinklayer makes contact with the metal wiring line within the first viahole.

According to the heterojunction bipolar transistor fabricating method ofthis invention, the heterojunction bipolar transistor capable ofreducing the heat radiation property and reducing the emitter inductanceis fabricated.

In an embodiment of the invention, after forming the first via hole, aninsulating film is so provided as to cover top surfaces and sidesurfaces of the emitter layer, the base layer and the collector layer,and the first via hole is furthermore extended toward the rear surfaceside of the substrate.

In the process of forming the first via hole, a long-time etchingprocess is performed in order for the first via hole to cut through theemitter layer, the base layer and the collector layer and to reach up toa specified depth within the substrate. Therefore, there occurs adimensional shift due to lateral expansion of the etching, which causesthe device processing accuracy to lower and characteristic variations tooccur. Also, there appears a rough surface in the inner wall of thefirst via hole i.e. rough side surfaces of the emitter layer, the baselayer and the collector layer. Particularly with the use of dry etching,plasma damage would be led to the etching surfaces. Therefore, there isa possibility of deterioration in device characteristics.

Thus, in the heterojunction bipolar transistor fabricating method ofthis embodiment, after forming the first via hole which extends throughthe emitter layer, the base layer and the collector layer, an insulatingfilm is so provided as to cover top surfaces and side surfaces of theemitter layer, the base layer and the collector layer. The first viahole is furthermore extended toward the rear surface side of thesubstrate. As a result, dimensional shifts of the first via hole due tothe etching are suppressed, so that higher device accuracy and highercharacteristic uniformization can be obtained. Further, occurrence ofsurface roughnesses and damage on the side surfaces of the emitterlayer, the base layer and the collector layer can be eliminated.Therefore, higher device reliability can be obtained.

In an embodiment of the invention, a wet etching process or a low-powerconditioned dry etching process is performed in the step of forming thefirst via hole, and a high-power conditioned dry etching is performed inthe step of extending the first via hole toward the rear surface side ofthe substrate.

In the heterojunction bipolar transistor fabricating method of thisembodiment, a wet etching process or a low-power conditioned dry etchingprocess is performed in the step of forming the first via hole, so thatoccurrence of rough surface and damage at the side surfaces of theemitter layer, the base layer and the collector layer can be effectivelyprevented. Therefore, higher device reliability can be obtained. Also, ahigh-power conditioned dry etching is performed in the step of extendingthe first via hole toward the rear surface side of the substrate, sothat a high-speed etching process can be achieved and the lateralexpansion due to etching is suppressed. Thus, the first via hole can bedeeply formed in a relatively short time.

In an embodiment of the invention, an undercut is formed by etching alower outer-edge portion of the emitter layer in the process ofpatterning the emitter layer; a metal film is deposited on the topsurface side of the substrate so as to form the surface electrode of thebase layer, with an inner edge of the surface electrode of the baselayer formed in self alignment to the emitter layer by using theundercut; and the metal film and the base layer are continuously etchedwith the same mask so that an outer edge of the surface electrode of thebase layer and an outer edge of the base layer become coincident witheach other.

In the heterojunction bipolar transistor fabricating method of thisinvention, since the first via hole extends through the emitter layer,the base layer and the collector layer, the individual layers and theirsurface electrodes surround the periphery of the first via hole in anelongate and annular form. Therefore, there is a possibility that thebase wiring resistance especially increases, which causes thehigh-frequency characteristics of the device to deteriorate. Althoughthe increase in base wiring resistance can be suppressed merely bybroadening the width of the base layer in order to broadening the areaof the surface electrode of the base layer, the base-collector capacityincreases and consequently the high-frequency characteristics islowered.

according to the heterojunction bipolar transistor fabricating method ofthis embodiment, an undercut is formed by etching a lower outer-edgeportion of the emitter layer in the process of patterning the emitterlayer, and a metal film is deposited on the top surface side of thesubstrate so as to form the surface electrode of the base layer, with aninner edge of the surface electrode of the base layer formed in selfalignment to the emitter layer by using the undercut. In addition tothis, in this embodiment, the metal film and the base layer arecontinuously etched with the same mask so that an outer edge of thesurface electrode of the base layer and an outer edge of the base layerbecome coincident with each other. As a result, the width of the surfaceelectrode of the base layer can be broadened fully to a range from theouter edge of the emitter layer to the outer edge of the base layerwithout broadening the width of the base layer. Consequently, increasesin the base wiring resistance can be suppressed while increases in thebase-collector capacity are avoided. Therefore, high-frequencycharacteristics of the device can be improved.

In an embodiment of the invention, the first via hole is formed afterforming the surface electrodes of the collector layer and the base layerand before forming the surface electrode of the emitter layer;simultaneously with time when the surface electrode of the emitter layeris formed, a wiring pattern of a same material as that of the surfaceelectrode is formed, the wiring pattern extending from a surface portionof the emitter layer to within the first via hole so as to reach abottom portion of the first via hole; and the metal wiring line isformed on the wiring pattern by a plating process.

In the heterojunction bipolar transistor fabricating method of thisembodiment, when forming the surface electrode of the emitter layer, thewiring pattern for plating the metal wiring line is simultaneouslyformed. Therefore, the process of the metal wiring line can be reduced,as compared with the case where the surface electrode of the emitterlayer and the metal wiring line are patterned independently of eachother. Accordingly, fabricating cost can be reduced.

In an embodiment of the invention, after forming the metal wiring line,the rear surface side of the substrate is not polished or the rearsurface side of the substrate is polished to a specified extent; asecond via hole is so formed as to extend from the rear surface side ofthe substrate up to the bottom portion of the first via hole; and theheat sink layer made of a metal is provided on the rear surface of thesubstrate so as to make contact with the metal wiring line within thefirst via hole through the second via hole.

According to the heterojunction bipolar transistor fabricating method ofthis embodiment, the polishing process or the rear surface of thesubstrate can be omitted, or needs only to be done to a small polishingextent.

In an embodiment of the invention, simultaneously with time when thefirst via hole is formed, an alignment hole deeper than the first viahole is formed from the top surface side of the substrate toward therear surface side of the substrate in a region other than regionsoccupied by the emitter layer, the base layer and the collector layer;the rear surface of the substrate is polished up to a bottom portion ofthe alignment hole; and a photolithography process for forming thesecond via hole is performed with reference to the alignment holeappearing on the rear surface side of the substrate.

In the heterojunction bipolar transistor fabricating method of thisembodiment, the second via hole to be formed from the rear surface sideof the substrate can be aligned with the first via hole formed from thetop surface side of the substrate with a normal aligner instead of anyspecial device such as a double-sided aligner. Therefore, high-accuracyalignment by normal photolithography techniques can be achieved.

In an embodiment of the invention, the second via hole is formed in aconical shape which broadens toward the heat sink layer on the substraterear surface, and an interior of the second via hole is buried with asame material as that of the heat sink layer.

Since the second via hole is formed in a conical shape which broadenstoward the heat sink layer on the substrate rear surface, and buriedwith a same material as that of the heat sink layer, the heat radiationpath substantially increases up to the heat sink layer, so that the heatradiation property can be further improved.

In an embodiment of the invention,

the surface electrodes of the collector layer, the base layer and theemitter layer are each formed by a lift-off process into a patternedsurface electrode which surround a periphery of a region where the firstvia hole is to be formed and a portion of which is cut out.

In the heterojunction bipolar transistor fabricating method of thisembodiment, patterns of the surface electrodes are generally annularwith part of each pattern cut out. Therefore, a solution of lift-offresist easily penetrates from outside to inside of the generally annularpatterns through the cutout portions. Thus, the lift-off process can beachieved more easily, as compared with the case where the surfaceelectrode patterns of the collector layer, the base layer and theemitter layer are completely annular patterns.

In an embodiment of the invention, in the process of polishing the rearsurface of the substrate up to the bottom portion of the first via hole,the polishing process is ended, by observing electric resistance of apolishing liquid, at a time point when cut chips of the metal wiringline within the first via hole mingle into the polishing liquid, causingthe electric resistance of the polishing liquid to show a change.

According to the heterojunction bipolar transistor fabricating method ofthis embodiment, since the end point of polishing process is determinedby change in electric resistance of the polishing liquid, the end pointof polishing process is clarified. Therefore, the accuracy of polishingextent on the rear surface side of the substrate is improved.

In an embodiment of the invention, a plurality of sets of the emitterlayer, the base layer and the collector layer of the heterojunctionbipolar transistor are arrayed on a common semiconductor substrate; andbefore the first via hole is formed in each heterojunction bipolartransistor, a device isolation region having a specified thickness isformed between the collector layers of adjacent heterojunction bipolartransistors by performing ion implantation.

Lamination of the emitter layer, the base layer and the collector layerof each heterojunction bipolar transistor is formed into a mesa shape inthis heterojunction bipolar transistor fabricating method. Therefore,when the photolithography process is performed to form the first viahole, the film thickness of the photoresist mask would become nonuniformdue to the mesa step gap, with the result that the photoresist maskbecomes thin in film thickness on the uppermost emitter layer. As aresult, there is a possibility that mask break may occur during theetching of the first via hole, causing the emitter layer to be etched.In addition, since merely thickening the film thickness of thephotoresist mask would cause the patterning precision to lower, thephotoresist mask cannot be thickened so much.

In the heterojunction bipolar transistor fabricating method of thisembodiment, therefore, a device isolation region having a specifiedthickness is formed between the collector layers of adjacentheterojunction bipolar transistors by performing ion implantation. Whenthe photolithography process for forming the first via hole, step gapsbetween the transistor portions and the field portions i.e. regionsbetween transistors on the substrate are reduced by virtue of thethickness of the device isolation region so that the photoresist mask isuniformized in film thickness. Therefore, during the process of etchingthe first via hole, the possibility of occurrence of mask break can beeliminated. Still, the successful coverage property of the transistorportions can be obtained during the formation of the metal wiring line,so that the device reliability can be enhanced.

It is desirable to use anti-activated ions such as oxygen ions, heliumions, hydrogen ions, as ions to be implanted for the formation of thedevice isolation region, in order that the device isolation region isformed into a high-resistance region.

In an embodiment of the invention, a high-frequency transmitter orreceiver includes, as a high-frequency amplifier, the heterojunctionbipolar transistor, the parallel connected heterojunction bipolartransistors or the heterojunction bipolar transistor made by theheterojunction bipolar transistor fabricating method as described above.

In the high-frequency transmitter or receiver, since the high-frequencyamplifier is superior in heat radiation property, high-power outputoperation with a high gain is enabled in high-frequency amplifications.Also, an enhanced reliability can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a planar pattern of an HBT according to a first embodimentof the present invention, and FIG. 1B shows a cross section taken alonga line B—B of FIG. 1A;

FIGS. 2A-2E are process diagrams for fabricating the HBT of FIG. 1;

FIG. 3 is a view for explaining heat dissipation paths in operation ofan HBT according to a second embodiment of the invention;

FIG. 4A shows a planar pattern of an HBT according to the secondembodiment of the invention, and FIG. 4B shows a cross section takenalong a line B—B of FIG. 4A;

FIGS. 5A-5F are process diagrams for fabricating the HBT of FIG. 4;

FIG. 6A shows a planar pattern of HBTs connected in parallel accordingto a third embodiment of the invention, and FIG. 6B shows a crosssection taken along a line B—B of FIG. 6A;

FIG. 7A shows a planar pattern of HBTs each having a regular hexagonalpattern and connected in parallel according to the third embodiment ofthe invention, and FIG. 7B shows a planar pattern of HBTs each having acircular pattern and connected in parallel;

FIG. 8 is a sectional view showing an HBT according to a fifthembodiment of the invention;

FIGS. 9A-9D are process diagrams for fabricating the HBT of FIG. 8;

FIG. 10 is a sectional view showing an HBT according to a sixthembodiment of the invention;

FIG. 11A shows a planar pattern in a case where an isolation groove isprovided between adjacent HBTs connected in parallel, the isolationgroove being buried with an interlayer insulator, and FIG. 11B shows across section taken along a line B—B of FIG. 11A;

FIG. 12A shows a planar pattern in a case where an isolation groove isprovided between adjacent HBTs connected in parallel, the isolationgroove being hollow, and FIG. 12B shows a cross section taken along aline B—B of FIG. 12A;

FIGS. 13A-13C are process diagrams showing an example in which a secondvia hole is formed from the substrate rear surface side by dry etchingin the fabrication of the HBT shown in FIG. 4;

FIGS. 14A-14F are process diagrams showing details of the processdiagrams of FIGS. 13A-13C;

FIG. 15 is a sectional view showing an HBT obtained by forming a secondvia hole from the substrate rear surface side by wet etching;

FIG. 16 is a sectional view showing an HBT according to a ninthembodiment of the invention;

FIG. 17 is a sectional view showing an HBT according to a tenthembodiment of the invention;

FIG. 18 is a sectional view showing an example in which a deviceisolation region is provided between adjacent HBTs;

FIG. 19 shows a planar pattern of an emitter ohmic electrode 4, a baseohmic electrode 5 and a collector ohmic electrode 6 applicable to theabove individual HBTs;

FIG. 20 shows a polishing apparatus applicable for polishing the rearsurface side of a substrate in the above individual embodiments;

FIG. 21A is a circuit diagram of a high-frequency two-stage amplifierequipped with any of the above HBTs, and FIG. 21B shows a portabletelephone on which the high-frequency two-stage amplifier is mounted;and

FIG. 22A shows a planar pattern of conventional HBTs connected inparallel, and FIG. 22B shows a cross section taken along a line B—B ofFIG. 22A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, the present invention is described in detail by way ofembodiments thereof illustrated in the accompanying drawings.

First Embodiment

FIG. 1A shows a planar pattern of an HBT (heterojunction bipolartransistor) 50 according to a first embodiment of the present invention,and FIG. 1B shows a cross section taken along a line B—B of FIG. 1A.

This HBT 50 has an emitter layer 1 made of n-GaAs or the like, a baselayer 2 made of p⁺-GaAs or the like, and a collector layer 3 made ofn-GaAs or the like, which are laminated on a top surface side of asemi-insulating GaAs substrate 13, as well as a plated heat sink layer(hereinafter, referred to as “PHS layer”) 12 made of metal and providedon a rear surface of the substrate 13.

The collector layer 3, the base layer 2 and the emitter layer 1 arelaminated in this order, as viewed from below, and fabricated into amesa state in a concentric rectangular pattern so that the area of thelayers 1, 2 and 3 may become smaller with increasing altitude of thelayers 1, 2 and 3. At the surfaces of the layers 1, 2 and 3, severalhundred nm thick surface electrodes 4, 5 and 6 for ohmic contact(hereinafter, referred to as “ohmic electrode”) are formed,respectively. WN film or WN/Ti/Au multilayer film is adopted for theemitter ohmic electrode 4, Pt film or Pt/Ti/Pt/Au multilayer metal filmis adopted for the base ohmic electrode 5, and AuGe film or AuGe/Ni/Aumultilayer film is adopted for the collector ohmic electrode 6. Althoughnot shown in FIG. 1A, these ohmic electrodes 4, 5, 6 are each patternedinto a rectangular frame shape.

A via hole 10 is provided through central portions of the emitter layer1, the base layer 2 and the collector layer 3. The via hole 10 has arectangular shape in cross section concentric with the layers 1, 2, 3.The via hole 10 extends, with constant cross-sectional dimensions, fromthe top surface of the emitter layer 1 to the rear surface of thesubstrate 13. A side wall 10 s of the via hole 10 is covered with aninterlayer insulator 9 in order to protect side walls of the layers 1,2, 3.

The emitter ohmic electrode 4 and the PHS layer 12 are connected to eachother with a metal wiring line 11 passing through in the via hole 10.Because the emitter ohmic electrode 4 and an opening of the via hole 10are very close to each other, the metal wiring line 11 is led from theemitter ohmic electrode 4 into the via hole 10 at a very short distance.

In this HBT 50, heat is generated during operation at junctions (mainly,an interface between the base layer 2 and the collector layer 3) on thetop surface side of the semiconductor substrate 13. The heat isdissipated through two paths. One of the paths is a path along which theheat conducts from the above-mentioned junction to the metal wiring line11 on the substrate top surface side by way of the emitter ohmicelectrode 4, and further conducts from the metal wiring line 11 withinthe via hole 10 to the PHS layer 12 on the substrate rear surface side.The other path is a path along which the heat conducts from thejunction, by way of the interior of the substrate 13 and the interlayerinsulator 9 on the via hole side wall 10 s, to the metal wiring line 11within the via hole 10, and further conducts from there to the PHS layer12 on the substrate rear surface side. Since the heat generated at thejunction is dissipated through two paths as described above, heatradiation property of the HBT 50 is improved. Also, since the metalwiring line 11 is led from the emitter ohmic electrode 4 into the viahole 10 at a very short distance, emitter inductance is reduced andhigh-frequency characteristics are improved, as compared with the casewhere a conventional air bridge is used.

This HBT 50 is fabricated as follows.

First, as shown in FIG. 2A, a collector layer 3, a base layer 2 and anemitter layer 1 of specified compositions and thicknesses are laminatedon top of a semi-insulating GaAs substrate 13 by epitaxial growthprocess (alternatively, a commercially available wafer on which theselayers 3, 2, 1 have already been laminated may be used). Next, theemitter layer 1, the base layer 2 and the collector layer 3 arepatterned by wet etching using an citric-acid-based or other etchingsolution or by RIE (Reactive Ion Etching) or other dry etching using achlorine-based gas or the like, respectively, in such a manner that thearea of layers 1, 2 and 3 becomes smaller with increasing altitude ofthe layers 1, 2 and 3. In this example, the length of each side of theemitter layer 1 is set to about 50 μm. In addition to this, as shown inFIG. 2B, ohmic electrodes 4, 5, 6 having rectangular frame patterns areformed at the surfaces of the layers 1, 2, 3 by lift-off technology,respectively.

Next, as shown in FIG. 2C, after a resist mask 7 is formed by performinga photolithography process, a via hole 10 is formed by performing an RIEor other dry etching process. The via hole 10 extends through centerportions of the emitter layer 1, the base layer 2 and the collectorlayer 3 and ends at a specified depth, for example, a depth of 100 μm.In this example, the width of each side of the emitter layer 1 is set toabout 4 μm. As a result, the emitter layer 1 is formed into arectangular shape having each side length of about 50 μm and a width ofabout 4 pim.

HBT has each side length of about 200 μm.

Next, as shown in FIG. 2D, an interlayer insulator 9 made of, forexample, SiN, SiO₂, SiON, polyimide resin or the like is deposited ontop of the substrate 13 uniformly in a thickness of several hundred nmby plasma CVD or other process so as to cover even an inner wall 10 s ofthe via hole 10. Subsequently, portions of the interlayer insulator 9present on the top surface of the emitter ohmic electrode 4 and on thebottom surface of the via hole 10 are removed by preformingphotolithography and etching processes so that the top surface of theemitter ohmic electrode 4 and the bottom surface of the via hole 10 areexposed.

Next, the material of a metal wiring line 11, for example, gold isuniformly deposited in about 10 μn thickness on top of the substrate 13by vapor deposition, sputtering, plating or other process so as to covereven the interlayer insulator 9 within the via hole 10. Then, a metalwiring line 11 is formed by patterning this gold so as to extend fromthe emitter ohmic electrode 4 to within the via hole 10 and to reach thebottom surface of the via hole 10.

Next, as shown in FIG. 2E, the rear surface side of the substrate 13 ispolished up to the bottom of the via hole 10 by CMP (Chemical MechanicalPolishing) process or the like. Then, an about 10 μm thick PHS layer 12made of, for example, gold is provided on the polished rear surface ofthe substrate 13 by vapor deposition, sputtering, plating or otherprocess so as for the PHS layer 12 to keep in contact with the metalwiring line 11 within the via hole 10.

After this, as shown in FIG. 1A, a Ti/Au lamination film having athickness of, for example, about 10 μm is deposited on top of thesubstrate 13 by vapor deposition, sputtering or other process. Bypatterning the lamination film, a base lead line 14 in contact with thebase ohmic electrode 5 is formed and a collector lead line 15 in contactwith the collector ohmic electrode 6 is formed.

In this way, the heat radiation property of HBT 50 is improved and theemitter inductance thereof is reduced.

Second Embodiment

FIG. 4A shows a planar pattern of an HBT 51 according to a secondembodiment of the present invention. FIG. 4B shows a cross section takenalong a line B—B of FIG. 4A. For an easier understanding, componentelements corresponding to the component elements of FIGS. 1A and 1B aredesignated by the same reference numerals (the same in the case of otherfigures).

A via hole 10 of this HBT 51 is formed into two stages which consist ofa first via hole 10 a and a second via hole 10 b. The first via hole 10a has constant cross sectional dimensions and extends through an emitterlayer 1, a base layer 2 and a collector layer 3. A second via hole 10 bhas constant cross-sectional dimensions smaller than those of the firstvia hole 10 a. The first via hole 10 a is cut through the emitter layer1, the base layer 2 and the collector layer 3, ending at a depth nearthe collector layer 3. The second via hole 10 b reaches from the bottomof the first via hole 10 a to the rear surface of a substrate 13. Theinterlayer insulator 9 is provided only on a side wall of the first viahole 10 a, and a side wall of the second via hole 10 b is in contactwith the metal wiring line 11. The rest of constitution is the same asin the HBT 50 of the first embodiment.

In this HBT 51, heat radiation property can be further improved, ascompared with that of the HBT 50 of the first embodiment. Asschematically shown in FIG. 3, heat is generated during operation atjunctions (mainly, an interface between the base layer 2 and thecollector layer 3) 27 on the top surface side of the semiconductorsubstrate 13. The heat is dissipated through two paths P1, P2. The heatdissipation of through the paths P1, P2 is the same as that in the firstembodiment. The one path P1 is a path along which the heat conducts fromthe junction 27 to the metal wiring line 11 on the top surface side ofthe substrate by way of the emitter ohmic electrode 4, and furtherconducts from the metal wiring line 11 within the via hole 10 to the PHSlayer 12 on the rear surface of the substrate. The other path P2 is apath along which the heat conducts from the junction 27 through theinterior of the substrate 13 to the metal wiring line 11 within the viahole 10, and the heat further conducts from there to the PHS layer 12 onthe rear surface of the substrate. The path P2 of this embodiment,unlike the first embodiment, allows the heat to conduct from thesubstrate 13 directly to the metal wiring line 11 without the interlayerinsulator 9. Therefore, heat radiation property of the HBT 51 in thisembodiment is further improved, as compared with that of the HBT 50 inthe first embodiment.

This HBT 51 is fabricated as follows.

First, as shown in FIG. 5A, like the first embodiment, a collector layer3, a base layer 2 and an emitter layer 1 are laminated in this order ontop of a semi-insulating GaAs substrate 13. The layers 1, 2 and 3 arepatterned so that an area of the layers 1, 2 and 3 may become smallerwith increasing altitude of the layers 1, 2 and 3. In this example, thelength of each side of the emitter layer 1 is set to about 50 μm. Inaddition to this, as shown in FIG. 5B, ohmic electrodes 4, 5, 6 havingrectangular frame patterns are formed at the surfaces of the layers 1,2, 3 by lift-off technology, respectively.

Next, as shown in FIG. 5C, after a resist mask 7 is formed by performinga photolithography process, a first via hole 10 a having a rectangularshape in cross section is formed by performing dry etching process suchas an RIE using chlorine-based gas or the like. The first via hole 10 aextends through center portions of the emitter layer 1, the base layer 2and the collector layer 3, and ends at a specified depth, which isseveral μm deeper than the depth of the collector layer 3. In thisexample, the length of each side of the first via hole 10 a is set toabout 40 μm. As a result, the emitter layer 1 is formed into arectangular shape having a side length of about 50 μm and a width ofabout 4 μm. In this first step at which the first via hole 10 a isformed, dry etching process is performed under the condition of very lowRF power, for example, about 10 W. As a result of this, the emitterlayer 1, the base layer 2 and the collector layer 3 can be effectivelyprevented from occurrence of surface roughnesses and damage on theirside surfaces. Therefore, the device reliability can be enhanced.

Next, as shown in FIG. 5D, an interlayer insulator 9 made of, forexample, SiN, SiO₂, SiON, polyimide resin or the like is deposited ontop of the substrate 13 uniformly in a thickness of several hundred nmby plasma CVD or other process so as to cover even an inner wall of thefirst via hole 10 a. Subsequently, a second via hole 10 b ending at aspecified depth, for example, a depth of about 100 μm within thesubstrate 13 is formed at a site several μm inward of the bottom of thefirst via hole 10 a by performing an RIE or other dry etching process.In this example, the second via hole 10 b is formed with the length ofeach side set to about 30 μm, concentrically with the first via hole 10a. As a result, the first via hole 10 a is extended toward the rearsurface of the substrate 13. In this second stage at which the secondvia hole 10 b is formed, dry etching process is performed under a highpower condition of, for example, 100 W RF power. As a result of this,high-speed etching can be executed while lateral expansion of etching issuppressed, so that a deep via hole 10 can be formed in relatively shorttime. Therefore, the device dimensional accuracy can be improved, andmoreover the number of fabricating steps can be reduced.

Next, as shown in FIG. 5E, portions of the interlayer insulator 9present on the top surface of the emitter ohmic electrode 4 and thebottom surface of the via hole 10 are removed by preformingphotolithography and etching processes so that the top surface of theemitter ohmic electrode 4 and the bottom surface of the via hole 10 areexposed.

After this, in the same way as the first embodiment, a metal wiring line11 is formed to extend from the emitter ohmic electrode 4 to within thevia hole 10 and each the bottom surface of the via hole 10. Further, asshown in FIG. 5F, the rear surface side of the substrate 13 is polishedup to the bottom of the via hole 10, and an about 10 μm thick PHS layer12 made of, for example, gold is provided on the polished rear surfaceof the substrate 13.

In this way, an HBT 51 capable of improving heat radiation property andreducing the emitter inductance is fabricated.

Third Embodiment

FIG. 6A shows a planar pattern of HBTs connected in parallel, and FIG.6B shows a cross section taken along a line B—B of FIG. 6A.

In this example, the HBTs 51 of the second embodiment are arrayed on acommon semiconductor substrate 13. A metal wiring lines 11, a basewiring line 14 and a collector wiring line 15 are electrically connectedto each other so as for adjacent HBTs 51 to enable parallel operations.Therefore, high-power operation is enabled. Also, heat generated atjunctions of the HBTs 51 is released to a PHS layer 12 on the substraterear surface every HBT. Accordingly, heat concentration due toperformance variations among the HBTs is suppressed so that thereliability is improved.

It is noted that, in this example, the metal wiring line 11, the basewiring line 14 and the collector wiring line 15 are formed of about 1 μmthick Au or Ti/Au or other lamination film so as to be suited forpatterning.

Fourth Embodiment

FIG. 7A shows HBTs 52 each having a regular hexagonal pattern andconnected in parallel, and FIG. 7B shows HBTs 53 each having a circularpattern and connected in parallel.

In the HBT 52 of FIG. 7A, an emitter layer 1′, a base layer 2′, acollector layer 3′ and a via hole 10′ extending through those layers 1′,2′, 3′ each have a regular hexagonal pattern. The rest of constitutionis the same as in the HBT 50 of the first embodiment. In this HBT 52,since the via hole 10′ has a cross-sectional configuration set to aregular hexagon in which apex angles are obtuse angles, electric fieldconcentration at peripheries of the via hole 10′ is suppressed.Therefore, the device reliability is improved. As a matter of course, ifthe cross-sectional configuration is a polygon in which apex angles areobtuse angles, similar functional effects can be produced.

Also, in the HBT 53 of FIG. 7B, an emitter layer 1″, a base layer 2″, acollector layer 3″ and a via hole 10″ extending through those layers 1″,2″, 3″ each have a circular pattern. The rest of constitution is thesame as in the HBT 50 of the first embodiment. In this HBT 53, since thevia hole 10″ has a cross-sectional configuration set to a circularshape, electric field concentration at peripheries of the via hole 10″is suppressed. Therefore, the device reliability is improved.

Fifth Embodiment

FIG. 8 shows a cross section of an HBT 54 according to a fifthembodiment of the present invention. This HBT 54 differs from the HBT 51of the second embodiment only in that an undercut 16 is formed at alower outer-edge portion of an emitter layer 1 and that a base ohmicelectrode 5 is formed in self alignment.

This HBT 54 is fabricated as follows.

When the emitter layer 1 is patterned according to the fabricatingprocess of the second embodiment, the emitter layer 1 is side-etched atits lower outer-edge portion to about 0.2 μm, by which an undercut (stepgap) 16 is formed as shown in FIG. 9A.

Next, as shown in FIG. 9B, metal films 5, 5′ such as Pt films orPt/Ti/Pt/Au lamination films with a thickness of about several hundrednm are deposited on top of the substrate 13 in order to make the baseohmic electrodes. As a result, inner edges of the base ohmic electrode 5are formed in self alignment to the emitter layer 1 by using the stepgap at the outer edge of the emitter layer 1. In this case, the metalfilm 5′ forms part of the emitter ohmic electrode 4. The metal films 5′is omitted in the illustration of FIG. 8.

Next, as shown in FIG. 9C, by performing a photolithography process,photoresist (mesa-etching mask) 7 for patterning of the base layer 2 isprovided so as to cover the emitter ohmic electrode 4 and the metal film5. Then, the metal film 5 and the base layer 2 are patterned bycontinuously performing dry etching processes such as RIE with the useof the same mask 7 so that the outer edge of the base ohmic electrode 5and the outer edge of the base layer 2 become coincident with eachother. As etching conditions for these processes, for example,sputtering etching conditions for Ar are adopted because the metal film5 contains a Pt layer at the stage of etching the metal film 5. Next, atthe stage of etching the base layer 2, conditions by making use ofchemical reaction with chlorine-based gas are adopted.

After this, the same processes as those in the second embodiment areexecuted to obtain an HBT as shown in FIG. 9D i.e. FIG. 8.

According to this HBT fabricating method, the width of the base ohmicelectrode 5 can be broadened fully to a range from the outer edge of theemitter layer 1 to the outer edge of the base layer 2 without broadeningthe width of the base layer 2. As a result, increases in the base wiringresistance can be suppressed while increases in the base-collectorcapacity are avoided. Therefore, high-frequency characteristics of thedevice can be improved.

Sixth Embodiment

FIG. 10 shows a cross section of an HBT 55 according to a sixthembodiment of the present invention. This HBT 55 differs from the HBT 54of the fifth embodiment only in that a metal wiring line 11 is formed byplating process on a wiring pattern 4′ made of material of the emitterohmic electrode 4. The emitter ohmic electrode 4 and the wiring pattern4′ are formed of, for example, several hundred nm thick WN film orWN/Ti/Au multilayer film.

For fabrication of this HBT 55, after the collector ohmic electrode 6and the base ohmic electrode 5 are formed and before the emitter ohmicelectrode 4 is formed, a first via hole 10 a is formed. Next, a wiringpattern 4′ is formed by vapor deposition or other process,simultaneously with formation of the emitter ohmic electrode 4. Thewiring pattern 4′ is made of the material of the emitter ohmic electrode4, and extends from the top surface of the emitter layer 1 to within thefirst via hole 10 a, reaching the bottom surface of the first via hole10 a. Then, with the wiring pattern 4′ used as a power-feeding metal, ametal wiring line 11 is formed on the wiring pattern 4′ by platingprocess. The rest of processes are executed in the same manner as in thefifth embodiment.

In this fabricating method, since the wiring pattern 4′ for plating themetal wiring line 11 is formed simultaneously when the emitter ohmicelectrode 4 is formed, the processes can be reduced, as compared withthe case where the emitter ohmic electrode 4 and the metal wiring line11 are patterned independently of each other. Therefore, the fabricatingcost can be reduced.

Seventh Embodiment

FIG. 11A shows a planar pattern in a case where an isolation groove 17is provided between HBTs 51 connected in parallel, and FIG. 11B shows across section taken along a line B—B of FIG. 11A.

In this example, like the third embodiment, a metal wiring lines 11, abase wiring line 14 and a collector wiring line 15 are electricallyconnected to each other so as for adjacent HBTs 51 to enable paralleloperations. Therefore, high-power operation is enabled. Also, heatgenerated at junctions of the HBTs 51 is released to the PHS layer 12 onthe substrate rear surface every HBT. Accordingly, heat concentrationdue to performance variations among the HBTs is suppressed so that thereliability is improved.

The isolation groove 17 that extends from top surface to rear surface ofthe substrate to partition adjacent HBTs 51 from each other is providedin a common semiconductor substrate 13, and the interior of theisolation groove 17 is buried with the interlayer insulator 9. Theisolation groove 17 is formed simultaneously with the via hole 10. Bythis structure, adjacent HBTs 51 are thermally shielded from each otherduring operation, thus never affecting each other, and moreover the HBTs51 are uniformized in heat capacity, thus operating uniformly.Therefore, the device reliability can be improved.

Similarly, FIG. 12A shows a planar pattern in a case where an isolationgroove 17 is provided between HBTs connected in parallel, and FIG. 12Bshows a cross section taken along a line B—B of FIG. 12A.

In this example, the isolation groove 17 is hollow with nothing buriedtherein. Like the foregoing example, adjacent HBTs 51 are thermallyshielded from each other during operation, thus never affecting eachother, and moreover the HBTs 51 are uniformized in heat capacity, thusoperating uniformly. Therefore, the device reliability can be improved.

It is also possible to bury a metal smaller in heat conductivity thanthe substrate 13 in the isolation groove 17. In such a case, heatgenerated at each HBT 51 conducts along the metal, being released to thePHS layer 12 on the substrate rear surface. Therefore, heat radiationproperty can be further improved and the device can be prevented fromdamage due to heat generation. As shown above, by providing theisolation groove 17, the device reliability can be enhanced.

Eighth Embodiment

FIGS. 13A to 13C show an example in which a second via hole 10 b havingconstant cross-sectional dimensions is formed from the rear surface sideof a substrate 13, differently from the fabrication of the HBT 51 shownin FIG. 4.

In this example, as shown in FIG. 13A, like the second embodiment, afirst via hole 10 a is formed, further an interlayer insulator 9 isformed, and thereafter a metal wiring line 11 is formed. Next, the rearsurface side of the substrate 13 is polished to a specified extent byCMP or the like so that the substrate 13 is thinned to a thickness of,for example, 100 μm Subsequently, as shown in FIG. 13B, the second viahole 10 b ranging from the rear surface side of the substrate 13 up tothe bottom of the first via hole 10 a is formed by performingphotolithography and RIE or other dry etching process. Then, as shown inFIG. 13C, by vapor deposition, sputtering, plating or other process, anabout 10 μm thick PHS layer 12 made of, for example, gold is provided onthe polished rear surface of the substrate so as to keep in contact withthe metal wiring line 11 within the first via hole 10 a through thesecond via hole 10 b.

More specifically, first, as shown in FIG. 14A, simultaneously when afirst via hole 10 a extending through the emitter layer 1, the baselayer 2 and the collector layer 3 and ending at a specified depth withinthe substrate 13 is formed, an alignment hole 29 deeper than the firstvia hole 10 a is formed from the substrate top surface side in a regionon the substrate 13 other than regions occupied by the layers 1, 2, 3.In this example, the depth of the alignment hole 29 is set to about 100μm. Next, as shown in FIG. 14B, a holding substrate 30 made of, forexample, Si is bonded to the top surface side of the substrate 13 withresin 31. In this process, the alignment hole 29 is buried with theresin 31. Next, by CMP or other process, the substrate 13 is polished atits rear surface side up to the bottom of the alignment hole 29, i.e.,until the resin 31 at the bottom of the alignment hole 29 appears on therear surface side of the substrate 13. As shown in FIG. 14C, byperforming a photolithography process by referencing the alignment hole29 (resin 31) that has appeared on the rear surface side of thesubstrate 13, a resist mask 32 for forming the second via hole 10 b issubstrate 13, the second via hole 10 b may also be formed with thepolishing process omitted.

FIG. 15 shows an example in which the second via hole 10 b is formed ina conical shape from the substrate rear surface side by wet etching withthe use of such etchant as an citric-acid base in place of the above RIEor other dry etching. An interior of the second via hole is buried withthe same material as that of the heat sink layer.

In this example, in the fabricated HBT 56, because the second via hole10 b broadens toward the PHS layer 12 on the substrate rear surface, theheat path to the PHS layer 12 substantially decreases. Therefore, heatradiation property can be further improved and the device reliabilitycan be further enhanced.

Ninth Embodiment

FIG. 16 shows a cross section of an HBT 57 according to a ninthembodiment of the present invention. This HBT 57 differs from the HBT 51of the second embodiment only in that the interior of the via hole 10 isfully buried with the same material as that of the metal wiring line 11.Such a metal wiring line 11 is formed by plating process.

In this HBT 57, since the interior of the via hole 10 is buried with thesame material as that of the provided on the rear surface side of thesubstrate 13. Next, as shown in FIG. 14D, by performing an RIE or otherdry etching process with this mask 32, a second via hole 10 b rangingfrom the rear surface side of the substrate 13 up to the bottom of thefirst via hole 10 a is formed. In this case, the etching process isended at a point at which the metal wiring line 11 at the bottom of thefirst via hole 10 a appears. Next, as shown in FIG. 14E, by vapordeposition, sputtering, plating or other process, an about 10 μm thickPHS layer 12 made of gold is provided on the polished rear surface ofthe substrate so as to keep in contact with the metal wiring line 11within the via hole 10 through the second via hole 10 b. Finally, asshown in FIG. 14F, the resin 31 is dissolved by a solvent, by which theholding substrate 30 is removed from the top surface side of thesubstrate 13.

In such a case, a second via hole 10 b to be formed from the rearsurface side of the substrate can be easily aligned by a normal alignerwith the first via hole 10 a that has been formed from the top surfaceside of the substrate. Therefore, high-accuracy positional alignment canbe achieved by normal photolithography techniques. Still, the polishingon the rear surface side of the substrate only needs to be done to asmall extent of polishing. In addition, without polishing the rearsurface side of the metal wiring line 11, a higher heat radiation effectthrough the via hole 10 can be obtained, so that the heat radiationproperty can be improved. As a result, stabler device characteristics aswell as higher device reliability can be obtained.

Tenth Embodiment

FIG. 17 shows a cross section of an HBT 58 according to a tenthembodiment of the present invention. This HBT 58 differs from the HBT 51shown in FIG. 4 only in that thickness of outer-edge portions 1 a, 1 bof the emitter layer 1 is set thinner than the thickness of the rest ofthe emitter layer 1, hence an edge-thinning structure.

For fabrication of this HBT 58, like the second embodiment, a first viahole 10 a is formed. Subsequently, by performing photolithographyprocess, on top of the rectangular-frame shaped emitter layer 1, arectangular-frame shaped resist mask (not shown) having a width narrowerthan the width of the rectangular frame of the emitter layer 1 isprovided. With this mask, the periphery of the emitter layer 1 ismesa-etched so that the thickness of outer-edge portions 1 a, 1 b of theemitter layer 1 becomes thinner than the thickness of the rest of theemitter layer 1 (edge-thinning structure). After this, like the secondembodiment, a base ohmic electrode 5 is formed. The rest of processesare executed in the same manner as in the second embodiment.

In this HBT 58, the thickness of the outer-edge portions 1 a, 1 b of theemitter layer 1 is set thinner than the thickness of the rest of theemitter layer 1, that is, what we called an edge-thinning structure isformed. Therefore, the edge-thinning structure prevents recombination ofholes and electrons generated between the peripheral edge portions 1 a,1 b of the emitter layer 1 and the base layer 2 during operation. As aresult, the device reliability can be enhanced.

Eleventh Embodiment

FIG. 18 shows an example in which a device isolation region 19 isprovided between adjacent HBTs 51. For simplicity, device isolationregions 19, 19 are shown on both sides of one HBT 51.

In this example, a plurality of sets of an emitter layer 1, a base layer2 and a collector layer 3 are patterned in arrays on a commonsemiconductor substrate 13. After ohmic electrodes 4, 5, 6 are formed onthe layers 1, 2, 3 and before the first via hole 10 a is formed, oxygenions, helium ions, hydrogen ions or the like are implanted at a highconcentration of, for example, about 1×10¹⁹ cm⁻² into the field regionbetween adjacent collector layers 3. As a result, a high-resistivity(specific resistivity: approx. 1×10⁷ Ω-cm) device isolation region 19having a specified thickness is formed in the field region. The rest ofprocesses are executed in the same manner as in the second embodiment.

In this case, during the photolithography process for forming the firstvia hole 10 a, step gaps between the transistor portions (regions of thelayers 1, 2, 3) and the field portions (regions between the transistors)on the substrate 13 are reduced by virtue of the thickness of the deviceisolation region 19. Thus, the photoresist mask is uniformized in filmthickness. Therefore, during the process of etching the first via hole10 a, the possibility of occurrence of mask break can be eliminated.Still, a successful coverage property for the transistor portions can beobtained during the formation of the metal wiring line 11, so that thedevice reliability can be enhanced.

Twelfth Embodiment

FIG. 19 shows a pattern of an emitter ohmic electrode 4, a base ohmicelectrode 5 and a collector ohmic electrode 6 applicable to the aboveindividual HBTs.

In this example, patterns of the emitter ohmic electrode 4, the baseohmic electrode 5 and the collector ohmic electrode 6 are those whichsurround a periphery of the region where the first via hole 10 a is tobe formed, with part of each pattern cut out. That is, the patterns ofthe ohmic electrodes 4, 5, 6, although being generally rectangular-frameshaped patterns, are not completely rectangular-frame shaped patternsbut those in which central portions 4 c, 5 c, 6 c on individual verticalsides of the patterns are cut out in the figure.

With the adoption of such patterns, the ohmic electrodes 4, 5, 6 areformed by the lift-off technology. When the lift-off resist is dissolvedwith a solution, the solution easily penetrates from outside to insideof the generally rectangular-frame shaped patterns through the cutoutportions 4 c, 5 c, 6 c. Thus, the lift-off process can be achieved moreeasily, as compared with the case where the patterns of the ohmicelectrodes 4, 5, 6 are completely rectangular-frame shaped patterns.

In addition, when regular hexagonal or circular patterns are basicallyinvolved as in the HBTs 52, 53 (FIG. 7) of the fourth embodiment, thepatterns of the ohmic electrodes 4, 5, 6 are set to those in which theregular hexagonal-frame shaped patterns or circular-frame shapedpatterns are partly cut out. In brief, the lift-off process can beeasily achieved only if the annular patterns are partly cut out.

Thirteenth Embodiment

FIG. 20 shows a polishing apparatus 20 applicable for polishing the rearsurface side of the substrate 13 in the above individual embodiments.This polishing apparatus 20 has, in its bath, a polishing table 23 onwhich a polishing target 22 (substrate 13 in this case) is to be placed.After the polishing table 23 is put into rotation, with a polishingliquid 21 thrown in, as the rear surface side of the substrate 13 ispolished, the polishing liquid 21 is changed into a liquid waste 26, andthe liquid waste 26 is stored in the bath. Electric resistance of thispolishing liquid waste 26 is observed by a resistance meter 24 equippedwith a resistance-forming sensor 25.

In the foregoing individual embodiments, when the rear surface side ofthe substrate 13 is polished up to the bottom of the first via hole 10a, electric resistance of the polishing liquid waste 26 is observed withthe resistance meter 24 having the resistance-forming sensor 25. Then,the polishing process is ended at a time point at which the electricresistance of the polishing liquid waste 26 is changed by mingling cutchips of the metal wiring line 11 in the first via hole 10 a with theliquid waste 26. With this arrangement, since the end point of polishingprocess is clarified, the accuracy of polishing extent on the rearsurface side of the substrate is improved.

Fourteenth Embodiment

FIG. 21A shows a circuit diagram of a high-frequency two-stage amplifier40 equipped with any of the HBTs indicated by reference numeral 34, 35in the foregoing embodiments.

This high-frequency two-stage amplifier 40 has a first-stageamplification HBT 34 for amplifying a signal inputted to an inputresistance 37 connected between an input terminal 33 and a ground 38,and a second-stage amplification HBT 35 for amplifying a signaloutputted by the HBT 34. An output of the HBT 35 is delivered to anoutput terminal 36. Since the HBTs 34, 35 are superior in heat radiationproperty, this high-frequency two-stage amplifier 40 is enabled toperform high-power output operation with a high gain in high-frequencyamplifications. Also, an enhanced reliability can be obtained. Forexample, this high-frequency two-stage amplifier 40 is mounted on aportable telephone 41 as a high-frequency transmitter or receiver, asshown in FIG. 21B. This portable telephone 41 is enabled to transmitmicrowaves with a high gain and with a large output power through anantenna 39.

In addition, the high-frequency amplifier is not limited to two-stageamplifiers, and a three-stage amplifier may be made up by providingthree HBTs.

What is claimed is:
 1. A heterojunction bipolar translator comprising:an emitter layer, a base layer and a collector layer laminated on a topsurface of a semiconductor substrate; and a heat sink layer made of ametal and provided on a rear surface of the substrate, wherein a viahole is cut through the emitter layer, the base layer, the collectorlayer and the substrate, and a surface electrode of the emitter layerand the heat sink layer are connected to each other by a metal wiringline running through within the via hole, wherein the metal wiring linedoes not form an air bridge between said surface electrode and said viahole.
 2. The heterojunction bipolar transistor according to claim 1,wherein the via hole has a cross section formed into a polygonal shapein which apex angles are obtuse angles, or a circular shape.
 3. Theheterojunction bipolar transistor according to claim 1, wherein aninterior of the via hole is buried with a same material as that of themetal wiring line.
 4. The heterojunction bipolar transistor according toclaim 1, wherein a peripheral edge portion of the emitter layer isformed so as to be thinner in thickness than residual portion of theemitter layer.
 5. A parallel connection of heterojunction bipolartransistors, wherein a plurality of heterojunction bipolar transistorsas defined in claim 1 are arrayed on a common semiconductor substrateand electrically connected to one another so as to be enabled to operatein parallel.
 6. The parallel connection of heterojunction bipolartransistors according to claim 5, wherein a groove extending from thetop surface of the substrate to the rear surface of the substrate isprovided in the common semiconductor substrate so as to partitionadjacent heterojunction bipolar transistors from one another.
 7. Ahigh-frequency transmitter or receiver including, as a high-frequencyamplifier, the heterojunction bipolar transistor as defined in claim 1.8. A high-frequency transmitter or receiver including, as ahigh-frequency amplifier, the parallel connected heterojunction bipolartransistors as defined in claim
 5. 9. A high-frequency transmitter orreceiver including, as a high-frequency amplifier, the heterojunctionbipolar transistor made by the heterojunction bipolar transistorfabricating method as defined in claim
 7. 10. A heterojunction bipolartransistor comprising: a semiconductor substrate; a heat sink layerprovided on a rear surface of the semiconductor substrate and made of ametal; a collector layer provided on a top surface of the semiconductorsubstrate; a base layer provided on the collector layer; an emitterlayer provided on the base layer; a metal wiring line running throughwithin a via hole cut through the emitter layer, the base layer, thecollector layer and the substrate; and an interlayer insulator providedbetween the metal wiring line and each of side walls of the emitterlayer, the base layer and the collector layer, said side walls definingthe via hole, wherein a surface electrode of the emitter layer and theheat sink layer are connected to each other by the metal wiring line,and wherein the metal wiring line does not form an air bridge betweensaid surface electrode and said via hole.